Thomas Roser and Ernest Courant (BNL)
1 The development of the proton synchrotron
The idea of pulsing the magnetic field to keep the circular orbit of the accel-erating particles unchanged was first proposed by M.O. Oliphant [Oliphant (1950)] in 1943. Oliphant recognized that maintaining stable orbits with changing magnetic holding field required longitudinal focusing or phase sta-bility, as described, for example, in [Wilson (1996)]. The principle of phase stability was soon after independently discovered by V.I. Veksler [Veksler (1944)] and E.M. McMillan [McMillan (1945)] and was first applied to the synchro-cyclotron and the electron ring synchrotron. But for higher energies the cyclotron would become too massive since it requires that the entire interior of the top energy orbit be filled with magnetic field. And, at the time, the energy of an electron synchrotron was limited to about 300 MeV by synchrotron radiation. This has since been increased substantially by using large rings and massive RF power. However, the best way of getting to the highest conceivable particle energies was then, and still is, the use of the much heavier protons. In a proton synchrotron the radio frequency has to be modulated with high precision so as to track the magnetic field and at the same time keep the orbit at a constant radius, but this complication is outweighed by the lack of synchrotron radiation.
Several proposals for proton synchrotrons appeared at about the same time, first for a 1 GeV machine at Birmingham, England, [Oliphant et al.
(1947)] in 1947 and for a 10 GeV machine at Berkeley [Brobeck (1947)]. Dis-cussions between Leland Haworth, Director of the newly formed Brookhaven National Laboratory, Ernest Lawrence, Director of the Radiation Laboratory (now the Lawrence Berkeley National Laboratory), and the Atomic Energy Commission (now the U.S. Department of Energy) led to the decision that both Brookhaven and Berkeley, instead of competing for the 10 GeV prize,
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Fig. 1. The Brookhaven Cosmotron with the injector van de Graaff in the foreground (courtesy of Brookhaven National Laboratory).
would each build a smaller proton synchrotron, one around 3 GeV and one at 6 GeV. Haworth chose the smaller size with the hope of getting it finished faster, but giving up on the possible discovery of the antiproton. In later years Haworth often said that this was the best decision he had ever made.
2 The first proton synchrotrons
All the first proton synchrotrons as well as the fixed field cyclotrons were
“weak focusing”, meaning that the magnetic bending field also provides a mainly constant focusing force on the beam in both horizontal and verti-cal planes [Blewett (1956); Green and Courant (1959)]. This is typiverti-cally expressed in terms of the guide field index
n=− ρ B0
∂By
∂r
r=ρ
(1)
Table 1. Main parameters of the first four weak focusing proton synchrotrons.
Birmingham Cosmotron Bevatron Synchrophasotron
Peak energy [GeV] 1 3 6.2 10
Injection energy [MeV] 0.46 3.6 9.9 9.0
Circumference [m] 28.27 69.63 120.16 208
Number of straight sections 0 4 4 4
Bending radius [m] 4.50 9.14 15.24 28
Field index 0.67 0.60 0.60 0.65
Magnetic field range [T] 0.02–1.26 0.03–1.38 0.03–1.54 0.02–1.30 RF frequency range [MHz] 0.30–9.70 0.36–4.18 0.36–2.47 0.18–1.44
Harmonic number 1 1 1 1
Rise time [s] 1 1 2 3.3
Cycle time [s] 10 5 6 12
Energy gain per turn [keV] 0.2 1.0 1.5 2.5
Number of RF stations 1 1 1 2
Peak excitation current [kA] 12.5 7.0 8.3 12.8
Peak stored energy [MJ] 7 12 80 148
Magnet cross section (H×V) [m] 2.44×2.44 2.38×2.38 6.25×2.90 7.5×5.3 Magnet gap (H×V) [m] 0.50×0.21 0.92×0.24 1.68×0.33 2.0×0.4
Weight of magnet steel [tons] 800 1650 9700 36000
Date of completion 1953 1952 1954 1957
wheren needs to satisfy 0< n < 1 to ensure beam stability. The betatron tunes are then equal to√
1−nand√
nfor the horizontal and vertical tunes, respectively. All of the first four machines, the Brookhaven Cosmotron, the Berkeley Bevatron, the Birmingham Synchrotron and the Dubna Syn-chrophasotron, were all very similar in their basic design and mainly differed in their size and therefore maximum energy that could be reached. The guide field indexnof their pulsed magnets was about 0.6. Table 1 shows the main parameters of these four machines.
The construction of the first proton synchrotrons required a very careful shaping of the magnetic field. The weak focusing made the orbit stability a significant concern especially since these would be the first machines where particles circulate for several seconds and travel for several hundred thousand kilometers during the acceleration cycle. Even small orbit disturbances could accumulate and lead to particle loss. To evaluate the necessary size of the required magnet aperture the Berkeley team decided to construct a scale model with one quarter the linear dimension of the final Bevatron [Lofgren (1950)]. The prototype machine successfully accelerated protons, injected at 0.7 MeV, to 6.5 MeV with just 0.3% efficiency but demonstrated a func-tioning injection process, a sufficiently low residual gas pressure and, most importantly, that a magnet gap of 0.3 by 1.7 m is acceptable for the Bevatron.
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All of the first proton synchrotrons ran with a harmonic number of one and the weak longitudinal focusing led to large radial synchrotron oscillations at the low injection energy adding to the need for a large horizontal aperture.
The large aperture also complicated the injection process and made it quite inefficient. All machines injected the beam using horizontal betatron painting and then moved the beam away from the inflector by ramping the magnetic field.
The weak focusing proton synchrotrons had successfully overcome the limitations of the cyclotrons and reached multi-GeV particle energies, which led to many new discoveries, most prominently the discovery of the antipro-ton at the Berkeley Bevatron. However, already when the first of them, the Brookhaven Cosmotron, started operation, it became clear that they have their own limitation of ever larger and more massive magnets as the beam energy increased. A new concept, the alternating gradient focusing, was needed that allowed for more compact magnets to reach even higher beam energies. The new concept was independently invented by Nicholas Christofilos in 1949 but without publishing it, instead he decided to issue a patent on the new invention [Christofilos (1949)], and Ernest Courant, Mil-ton S. LivingsMil-ton, Hartland Snyder and J. Blewett [Courant et al. (1952);
Courantet al. (1953)].
References
Blewett, J.P., The proton synchrotron,Rep. on Progress in Physics19(1956) 37.
Brobeck, W.M., Design study for a 10-BeV magnetic accelerator,Rev. Sci. Inst.19(1948) 545.
Christofilos, N. C. (1949). Focusing System for Ions and Electrons. US Patent No.
2,736,799.
Courant, E.D., Livingston, M.S., Snyder, H.S., The strong-focusing synchrotron, Phys.
Rev.88(1952) 1190.
Courant, E.D., Livingston, M.S., Snyder, H.S., Blewett J., Origin of the strong-focussing principle,Phys. Rev.91(1953) 202–203.
Green, G.K., Courant, E.D., The proton synchrotron,Encyclopedia of Physics, edited by Creutz, E., published by Springer Berlin Heidelberg, Volume 8/44 (1959) 218.
Lofgren, E.J., The proton synchrotron,Science111(1950) 295.
McMillan, E.M., The synchrotron — a proposed high energy accelerator,Phys. Rev. 68 (1945) 143.
Oliphant, M.O., The acceleration of particles to very high energies, unpublished manuscript (1943) University of Birmingham, U.K.
Oliphant, M.O., Gooden, J.S., Hide, G.S. The acceleration of charged particles to very high energies,Proc. Phys. Soc.59(1947) 666.
Veksler, V.I., A new methode of accelerating relativistic particles, Comptes Rendus de l’Academie Sciences de L’URSS,43,8(1944) 329.
Wilson, E.J.N., Fifty years of synchrotrons, EPAC96.
Chapter 8